My grown wolfhounds get Iams or Eukanuba dry food for large breeds, and the puppy gets the puppy version. It has to be said that, like human children, they love the 'junk food' of 'meaty chunks', but I only give that as an occasional treat on top of the regular meals.
You have to feed something which is formulated for the particular needs of a large breed like a St Bernard. This is particularly important when they are puppies; which in their case means far older than 6 months; because they grow so fast.